[Geojson] coordinate order

[Tim Schaub]

Hey-

See the updated GeoJSON spec v5
http://wiki.geojson.org/GeoJSON_draft_version_5

We had a discussion on IRC today about geometry coordinates.  We all 
agree that we want to support more than two dimensions.  We also want 
simple clients to be able to know which dimensions at least the first 
two elements in a coordinates array refer to.

Regarding more than two dimensions, the spec talks about two dimensions 
for point geometry and goes on to say "any number of additional 
dimensions are allowed, and interpretation and meaning of these 
coordinates is beyond the scope of this specification."

Does this sit well?  We say you've got to have two elements in the 
coordinates array but more are allowed.

Regarding the order of elements in the coordinates array for GeoJSON 
geometries, we say that the first two elements are in x, y order (lon, 
lat for dd).  This is what we have said from the start.

The change to the spec is to accommodate CRS that define a different 
coordinate order.  Instead of requiring all clients to know about all 
CRS coordinate order conventions, we require that GeoJSON geometries 
referencing a CRS that defines non-xy coordinate order include a 
"coordinate_order" member in the CRS object.

This means a point geometry that references EPSG:4326 would look like this:

{
    "type": "Point",
    "coordinates": [-180.0, 90.0],
    "crs": {
        "type": "EPSG",
        "properties": {"code": 4326},
        "coordinate_order": [1, 0]
    }
}

The benefits of this are that it allows all clients simple access to the 
proper dimensions in the geometry coordinates array and allows smarter 
clients who are strict about CRS coordinate order to map to the correct 
dimension in our coordinates arrays.

The drawbacks of this are that people like to argue that the EPSG has 
the right to assign meaning to any sequence of numbers in a data 
structure that references EPSG.  I say that is fine, they get access to 
our coordinate_order array.  If the EPSG says that the first value in a 
sequence refers to the northing of a point, then they look at the first 
value in our coordinate_order array and know which element to use from 
our coordinates array.

Makes things a bit more complex for clients who know about CRS and 
simpler for the rest.

I think this thing is ready to let out into the wild.
Tim